One of the goals of cosmetic science has long been the development of products capable of increasing the actual and/or perceived smoothness of human skin. Such skin smoothing compositions have traditionally contained physical exfoliants such as luffa and apricot kernel paste which, when rubbed against the skin, facilitate the rapid dislodging of debris and dead cells from the skin surface. The dislodged material is then removed from the skin surface along with the spent product, for example by the action of wiping, rinsing or washing, leaving behind a smoother, cleaner skin surface.
A more recent phenomenon in cosmetic science has been the development of products containing hydroxyacids; see C. D. Wheeler, Soap/Cosmetics/Chemical Specialties, 71 (2), 24 (1995). Hydroxyacids have been shown to be useful in the topical treatment of a variety of dermatological disorders, such as ichthyosiform dermatoses, dry skin and acne. While the exact mechanism of action of hydroxyacids on the skin is not known, it is generally believed that these materials are able to accelerate skin cell turnover by chemically exfoliating the dead cells on the skin surface over a period of days or weeks.
The ready availability of hydroxyacids from natural sources, such as citrus fruits, sugar cane and sour milk, as well as their favorable safety and efficacy profiles, have contributed to the widespread acceptance of hydroxyacid-containing products by consumers. Consumers have also come to expect and demand that such products be chemically and physically stable for long periods of time, easily applied to the skin, non-irritating and aesthetically pleasing upon application, in addition to being safe and efficacious.
Hydroxyacids commonly employed in cosmetic products may be broadly classified as being either alpha-hydroxyacids, wherein the hydroxyl group is situated on a carbon adjacent to the carbonyl carbon, or beta-hydroxyacids, wherein the hydroxyl group is situated on a carbon which is two carbons away from the carbonyl carbon. The most commonly used alpha-hydroxyacids in cosmetic products are those having a relatively small carbon backbone and low molecular weight, especially lactic acid (2-hydroxypropanoic acid), glycolic acid (hydroxyacetic acid), citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) and tartaric acid (2,3-dihydroxybutanedioic acid). The most commonly used beta-hydroxyacid in cosmetic products is salicylic acid (2-hydroxybenzoic acid).
It would be highly advantageous to exploit the chemical exfoliant properties of hydroxyacids in skin smoothing compositions containing physical exfoliants. Unfortunately hydroxyacids, and in particular the low molecular weight alpha-hydroxyacids referred to hereinabove, are significantly soluble in water and aqueous mixtures by virtue of their polar hydroxyl and carboxyl groups. Consequently, the long-term benefits which may be achieved by the use of hydroxyacids in skin smoothing compositions are limited by the facility with which the hydroxyacids are removed from the skin surface in the course of wiping, rinsing or washing. This is a particularly crucial drawback when low molecular weight hydroxyacids are formulated into skin smoothing compositions formulated with high levels of skin-cleansing surfactants, which may further increase the water solubility of the hydroxyacids.